There is no comparison, the Asus board is miles ahead better than the Gigabyte but it is an unfair comparison since the price difference between them is huge. Since you're only going for a mild overclock, the Gigabyte Z77N-WIFI is sufficient and gives you savings which could go to a GTX660 Ti or HD7870.

The amount of electricity saved from a more efficient PSU must be justified for its extra cost. For most servers where they are turned on 24/7, the savings is apparent but not for the normal home PC. You could get it if you want to but I would suggest something cheaper, like an 80+ Silver/Gold from Silverstone or Seasonic.

I think you heard Linus incorrectly. He probably meant LGA1155 in relation to LGA1156, not LGA1155 in relation to upcoming Intel processors. The upcoming Haswell/Broadwell will only work on LGA1150, thus making LGA1155 an impractical choice if there is a necessity to upgrade. I doubt that you will need an upgrade that soon in a span of 2-4 years because the Core i5 3570K is beastly for a mITX form factor. Should you find performance underwhelming, bump up the clocks.

The only suitable AIO watercooling unit is ones that are single 120mm and thin. You're restricted to a H55/H60. The H80 and H100 is impossible to fit in that case without modding.

Thanks mate, very thorough answer, I like that.

Now the socket question was not for this mITX build but for a friends ATX build. Gonna assemble some parts and the only sockets I can see available that have i7 processors like 3770k and 3930 are LGA 1155 and 2011. Which one of those is most likely to be futureproof so to speak? I'm guessing the Haswell/Broadwell are upcoming projects and there's no socket yet(?) so there's no point lingering on that.

For my mITX I wanted a cooling solution that wouldn't take too much space but still be efficient in case I decide to OC a tiny bit, or maybe more.

Also you say the Asus board is miles ahead compared to the Gigabyte one. Could you specify this? I'm very curious to know what actually differs apart from some I/O ports. I see that the Asus one supports higher freq RAM on the same chipset.

Lastly about the PSU, yes I figured it might be a slight overkill to go for a 80 PLUS Platinum seen as how I am not gonna use it 24/7. What do you think about the Be Quiet Straight Power E9 480W CM?
I'm searching on a swedish website called "prisjakt.nu" which compares prices on the swedish market and try to find a cheap but good PSU.

The i5's and i7's of gen 3 have a TDP of 77W while i've seen some charts go as high as 130ish W. The GTX 660 has about 150 if not mistaken so let's count up to 300 on those two. Still leaves plenty of room for other things like an SSD and second HDD. More than that won't be put in this case anyway. I'm not sure how much the MB and RAM needs though.